1. Field of the Invention
The present invention relates to the package of a printed circuit board and in particular to a method and a structure for preventing increased thermal expansion between a display panel and a printed circuit board.
2. Description of the Related Art
Printed circuit boards can electrically connect panels via flexible printed circuit boards (FPCs). For example, a liquid crystal display module (LCM) comprises a glass substrate and a PCB (Printed Circuit Borad) connected thereto via a FPC. In a LCD fabrication process, electrodes on the PCB of the LCM are electrically connected to the electrodes of the FPC by thermal bonding.
FIG. 1A is a schematic diagram of a LCM. Referring to FIG. 1A, the LCM 100 comprises a PCB 10, a plurality of FPCs 12 and a glass substrate 13 connected to the FPC 12. A plurality of electrodes 14 are disposed on the inner side of the PCB 10 and in the thermal bonding region 11. Each FPC 12 comprises a plurality electrodes corresponding to the electrodes 14 in the thermal bonding region 11 of the PCB 10. The FPCs 12 are connected to the glass substrate 13 of the LCD on the opposite side thereof(not shown). As shown in FIG. 1B, when the FPC 12 and the PCB 10 are bonded, the electrodes on both are electrically connected. Therefore, signals can be transmitted from the PCB 10 to the LCD 13 through the FPC 12 to display images.
Typically, the FPCs 12 are bonded to the PCB 10 by thermal bonding, wherein a long press pad or a short press pad is employed. Alignment of the PCB 10 and the FPC 12 is affected, however, by thermal expansion and cold shrinkage during bonding. FIG. 2A is a top view of a LCM. Typically, thermal expansion of the PCB material 10 is greater than material of the FPC 12. Consequently, the FPC 12 is deformed according to the thermal expansion of the PCB 10. Additionally, the thermal expansion effect increases from the center to the edge of the PCB 10. The FPC 12 adjacent to the center line of the PCB 10 is pressed and the effect is increased to the edge. Thus, the square shape of the FPC 12 is deformed to a ladder-shape.
Additionally, as shown in FIG. 2B, the LCM 100 shrinks when cooling subsequent to thermal bonding. Thus, the FPCs 12 are again deformed by shrinkage of the PCB 10. Specifically, the deformation of the FPCs 12 near the edge of the PCB 10 is increased, thus, causing the FPCs 11 to break.
Consequently, compensation for thermal expansion must be considered. The deformation of the FPCs 12 varies, however, according their position, hence determining a compensation factor is difficult.